George Zimmerman glances back at the back of the courtroom. Photograph: Pool/Getty Images

Swiftly but methodically, prosecutors have begun building their case against George Zimmerman, charged with second-degree murder in the shooting of Trayvon Martin, an unarmed black teenager. For their part, defence attorneys have been doing their dogged best, sometimes through hectoring cross examinations, to sow sufficient seeds of reasonable doubt to get their client acquitted.

Martin, who had just turned 17, was returning from a convenience store to the town house where his father was staying on a rainy night on 26 February 2012. Zimmerman, a member of the same gated community's neighbourhood watch, was following Martin – first in his car and then on foot – despite the advice of a police dispatcher he had called to report the apparent intrusion. During the first week of testimony police witnesses, in customary scene-setting in American murder trials, described finding Martin's body shot through the heart. The officers were followed on the stand by neighbouring residents of the Retreat at Twin Lakes community who offered fragmentary, conflicting and sometimes confusing descriptions of the fatal altercation between Martin and Zimmerman, and of the indistinguishable screams for help they heard from a distance.

The prosecutors' goal at this stage is to construct a strong, simple narrative for the jury of six women, five whites and one Hispanic. In that scenario, Martin was innocently returning home when he was accosted by Zimmerman. A fight, initiated by Zimmerman, resulted in Martin's needless and tragic death.

In his opening statement, assistant state attorney John Guy promised the jury:

You will know in your head, in your heart and in your stomach that George Zimmerman didn't shoot Trayvon Martin because he had to. He shot him for the worst of all reasons, because he wanted to.

For its part, the defence's Don West argued in his lengthy opening statement that, "George Zimmerman is not guilty of murder. He shot Trayvon Martin in self-defence after being viciously attacked."

The most compelling witness of the week was Rachel Jeantel, 19, a friend of Martin's from Miami who recounted her mobile phone conversation with him in the last moments of his life. Jeantel testified that Martin told her he was being stalked in the dark by a "creepy-assed cracker" (a pejorative term for rural Southern whites), although Zimmerman is half Hispanic. Jeantel said she overheard Martin asking the man, "What are you following me for?" before the call ended with the sounds of a struggle.

The defence, which will present its case in full when the prosecution is finished, then tried to pick apart Jeantel's account. West's questions, launched like a virtual swarm of gnats, focused on inconsistencies in Jeantel's previous statements, and were designed to impeach her credibility and lessen the impact. In the end, West succeeded only in goading Jeantel into responding with angry outbursts and sullen, monosyllabic answers – but never backing away from the basic elements of her testimony.

In presenting its case, the prosecution also appears to be setting some snares for the defence, in particular relating to the position of Trayvon Martin's body, found on his back, which they say is at variance with Zimmerman's accounts of the struggle. One witness seemed to identify Zimmerman as the figure straddling Martin after the shooting. This contradicts defence claims that Martin was atop a supine Zimmerman, punching him in the nose and beating his head against a concrete pavement, when the fatal shot was fired. But they have yet to explain how, under those circumstances, Zimmerman was able to reach under Martin's legs and withdraw his pistol from the holster on his hip. Zimmerman's account, one prosecutor said, amounted to little more than "a tangled web of lies".

As a result, one effect of the first week's testimony is that the defence is now almost compelled to put their client on the witness stand to refute the prosecution's unfolding scenario.

The most critical challenge for both sides will be framing the fight for the jury, to determine which of the men was justifiably "standing his ground" under Florida law.

If the confrontation began with Zimmerman's profiling the hoodied youth, stalking him and then approaching him with a hostile intent and initiating the struggle, then Martin could be reasonably seen to be acting in self-defence, in fear for his life. But if the time frame is more narrow, beginning moments later, with the fight going badly for Zimmerman, then his attorneys can argue that he was acting in fear for his life.

In the days following Martin's shooting, Sanford police did not hold or charge Zimmerman. Then, Florida governor Rick Scott, acting in response to political pressure generated by marches, rallies, demonstrations and editorials, in Sanford and across the country, appointed a special prosecutor who indicted him on second-degree murder charges.

Predicting any jury verdict is chancy, especially after only a week of testimony. But in this case gaining a second-degree murder conviction may be especially problematic. Under political pressure, some observers believe, prosecutors may have miscalculated in charging Zimmerman with second-degree murder. In doing so, they may have overreached, given that second-degree murder requires proof of "ill will" or "hatred" and "an indifference to human life".

A lesser charge, voluntary or involuntary manslaughter, carries a lighter burden of proof, only that the accused acted with "culpable negligence", in a manner that demonstrated a reckless disregard for human life. However, if the defence thinks their case is going badly, they can request that presiding judge Debra Nelson offer jurors the alternative of finding Zimmerman guilty of the lesser charge.

Although Sanford's African-American community mobilized 16 months ago, there has been very little so far in the way of demonstrations during the trial. Local activists blame a court order restricting gatherings to a treeless green enclosure in front of the courthouse. Despite the brutal central Florida summer sun and frequent afternoon rain storms, the judge's order prohibits umbrellas and even water bottles by demonstrators.